Economizer target temperature shift during mechanical cooling

ABSTRACT

A method and apparatus for cooling a supply of air is provided. A unit controller receives a cooling demand signal. In response to the cooling demand signal, the unit controller adjusts an economizer according to an economizing function. The economizing function is configured to achieve an initial free cooling target temperature as a discharge air temperature. The unit controller receives an additional cooling demand signal. In response to the additional cooling demand signal, the unit controller reduces the free cooling target temperature. The unit controller adjusts the economizer according to the economizing function, with the economizing function configured to achieve the reduced free cooling target temperature as the discharge air temperature. Additionally in response to the additional cooling demand signal, the unit controller signals a unit to perform mechanical cooling on a supply of air received from the economizer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/165,069, filed on Jan. 27, 2014. U.S. patent application Ser. No.14/165,069 is incorporated herein by reference.

TECHNICAL FIELD

This application relates to HVAC controllers and, more particularly, tocontrol of HVAC economizers.

BACKGROUND

One function of a Heating, Ventilation, and Air Conditioning (HVAC) unitis providing conditioned air to cool an enclosed space, usually abuilding. During this conditioning, a volume of air is drawn into theHVAC unit, conditioned, and discharged into the building. Typical HVACunits can perform both free cooling and mechanical cooling. Free coolingutilizes the cooler outdoor air as some or all of the air volume.Mechanical cooling is the use of mechanical components to reduce thetemperature of the air volume.

An evaporator coil is a conventional component in mechanical cooling.The evaporator coil contains a refrigerant. A blower moves air past theevaporator coil, transferring heat from the air to the evaporator coil.During mechanical cooling, the air volume received by the evaporatorcoil can be called the “supply air” for mechanical cooling.

Free cooling is preferable to mechanical cooling because free coolinguses substantially less energy. During free cooling, the HVAC unitcontroller performs an economizing function. The economizing functionattempts to make the temperature of the air discharged from the HVACunit (the “discharge air temperature”) equal to a target temperature(the “free cooling target temperature”). The economizing functionincreases or decreases the amount of outdoor air drawn into the HVACunit as necessary to achieve the free cooling target temperature.

Free cooling alone may not be enough to satisfy the cooling demands of abuilding. If this is the case, the HVAC unit can perform free coolingand mechanical cooling together. However, combining free cooling andmechanical cooling creates difficulties with the amount of outdoor airthat should be used as the supply air.

Some HVAC unit controllers continue the economizing function when freecooling and mechanical cooling are combined. In practice, these unitcontrollers do not receive the benefits of the free cooling. Mechanicalcooling tends to quickly reduce the discharge air temperature below thefree cooling target temperature. As a result, the economizing functionstops utilizing any outdoor air, and the HVAC unit effectively uses onlymechanical cooling.

Other HVAC unit controllers instead utilize only outdoor air as thesupply air when combining free cooling and mechanical cooling. Theseunit controllers can produce undesirably cold discharge air. Forexample, the outdoor air may be 50 degrees Fahrenheit, but the dischargeair may be below 32 degrees Fahrenheit after being cooled by theevaporator coil. The resulting air delivered from the HVAC unit may beuncomfortable for the occupants of the building because it is too cold.

This undesirably cold air may also create conditions where theevaporator coil freezes. The cold refrigerant inside the coil can chillthe outside surfaces of the coil to below the freezing point of themoisture content of the air. The moisture droplets in the air passingover the evaporator coil may then freeze onto the coil. The iceformation blocks off air flow paths through the coil, resulting inreduced air flow into the building. The frozen evaporator coil may causethe air delivered from the HVAC unit to be uncomfortably humid inaddition to being uncomfortably cold.

It would be desirable if an HVAC unit could effectively utilize freecooling and mechanical cooling simultaneously without the risk ofundesirably cold discharge air. In particular, the evaporator coilshould preferably not cool the discharge air below the freezing point ofwater.

SUMMARY

In an embodiment, a method of cooling a supply of air is provided. Aunit controller receives a cooling demand signal. In response to thecooling demand signal, the unit controller adjusts an economizeraccording to an economizing function. The economizing function isconfigured to achieve an initial free cooling target temperature as adischarge air temperature. The unit controller receives an additionalcooling demand signal. In response to the additional cooling demandsignal, the unit controller reduces the free cooling target temperature.The unit controller adjusts the economizer according to the economizingfunction, with the economizing function configured to achieve thereduced free cooling target temperature as the discharge airtemperature. Additionally in response to the additional cooling demandsignal, the unit controller signals a unit to perform mechanical coolingon a supply of air received from the economizer.

DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following DetailedDescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 depicts an HVAC unit and an HVAC controller which may benefitfrom an exemplary embodiment of the present invention; and

FIG. 2 depicts a method that an HVAC controller may perform to avoid theproblem of undesirably cold discharge air when utilizing outdoor air assupply air.

DETAILED DESCRIPTION

In the following discussion, numerous specific details are set forth toprovide a thorough explanation. However, such specific details are notessential. In other instances, well-known elements have been illustratedin schematic or block diagram form. Additionally, for the most part,specific details within the understanding of persons of ordinary skillin the relevant art have been omitted.

With reference to FIG. 1, depicted are a Heating, Ventilation, and AirConditioning (HVAC) unit 100 and a controller 102. Unit 100 containsmechanical components which, among other things, can cool air flowingthrough the unit. This air in turn cools a building. Unit 100 can alsosense air temperatures related to cooling the building. Generallyspeaking, a building thermostat or building energy management systeminforms controller 102 on the cooling needs of the building, andcontroller 102 instructs unit 100 on how to respond to those coolingneeds. Unit 100 may be referred to as a Roof-Top Unit (RTU). However,unit 100 is not necessarily located on a rooftop.

Unit 100 has economizer 104. Economizer 104 comprises outdoor air damper106 and return air damper 108. Outdoor air damper 106 can receive airfrom outside the building (“outside air flow”), and return air damper108 can receive air returned from inside the building (“return airflow”). Outdoor air damper 106 and return air damper 108 may each beopened, to receive air from their respective sources, or closed, to keepout air from their respective sources.

Unit 100 also has indoor evaporator coil 110. During mechanical cooling,the refrigerant in indoor evaporator coil 110 absorbs heat from the airflow as is known in the art. The combination of air from outdoor airdamper 106, if any, and return air damper 108, if any, forms the air formechanical cooling.

Unit 100 has outdoor air temperature (OAT) sensor 112, return airtemperature (RAT) sensor 114, and discharge air temperature (DAT) sensor116. OAT sensor 112 measures the temperature of the outdoor air, RATsensor 114 measures the temperature of the return air, and DAT sensor116 measures the temperature of the discharge air. Controller 102receives the temperatures measured by sensors 112, 114, and 116.

Free cooling is available when the outdoor air temperature, measured byOAT sensor 112, is at or below a user-adjustable temperature. Duringfree cooling, controller 102 performs an economizing function. Theeconomizing function adjusts economizer 104 to achieve the free coolingtarget temperature as the discharge air temperature. The free coolingtarget temperature may be 55 degrees Fahrenheit.

By adjusting economizer 104, controller 102 can change the relativeamounts of cooler outdoor air and warmer return air that pass throughHVAC unit 100. When the discharge air temperature is at or above thefree cooling target temperature, controller 102 opens outdoor air damper106 fully and closes return air damper 108 fully in an attempt to lowerthe discharge air temperature. When the discharge air temperature isbelow the free cooling target temperature, controller 102 graduallycloses outdoor air damper 106 and gradually opens return air damper 108in an attempt to raise the discharge air temperature.

As previously discussed, some conventional controllers continue theeconomizing function during both free and mechanical cooling. Inpractice, these controllers tend to experience the discharge airtemperature dropping below the free cooling target temperature due tothe mechanical cooling. These controllers would then close outside airdamper 106 in an attempt to warm the discharge air up to the freecooling target temperature. Other controllers would fully open outsideair damper 106 during the mechanical cooling, which risks the problem ofundesirably cold discharge air. With reference to FIG. 2, depicted is amethod 200 that controller 102 may perform to avoid the problem ofundesirably cold discharge air when utilizing outdoor air as supply air.At 202, controller 102 receives an initial cooling demand when theoutdoor air is suitable for free cooling. This cooling demand may be asignal may be sent from a thermostat or building energy managementsystem. At 204, controller 102 responds to the cooling demand with freecooling. Controller 102 performs the economizing function, adjustingeconomizer 104 as necessary to maintain the discharge air temperature,as measured by discharge air temperature sensor 116, at the free coolingtarget temperature.

At 206, controller 102 receives an additional cooling demand, whichagain may be a signal sent from the thermostat or energy managementsystem. This additional cooling demand indicates free cooling isinsufficient to cool the building. At 208, this combination of theadditional cooling demand and the availability of free cooling causescontroller 102 to reduce the free cooling target temperature to asecond, reduced free cooling target temperature. 208 may occursimultaneously with 210.

The reduced free cooling target temperature is lower than the originalfree cooling target temperature. The reduced free cooling targettemperature may be 45 degrees Fahrenheit, but other temperatures may beused. A reduced free cooling target temperature lower than 45 degreesFahrenheit may be suitable, although a reduced free cooling targettemperature which is too low can cause an evaporator coil freezingissue.

At 210, controller 102 signals unit 100 to perform mechanical cooling.Controller 102 may send a signal or signals to unit 100 that instructsunit 100 to perform mechanical cooling. Unit 100 also continues toperform free cooling, but the economizing function now attempts tomaintain the discharge air temperature at the reduced free coolingtarget temperature. Due to the reduced free cooling target temperature,controller 102 gradually closes outdoor air damper 106 and graduallyopens return air damper 108 when the discharge air temperature is belowthe new reduced free cooling target temperature, not the original freecooling target temperature. When the discharge air temperature is at orabove the reduced free cooling target temperature, controller 102 opensoutdoor air damper 106 gradually and closes return air damper 108gradually in an attempt to lower the discharge air temperature.

At 212, controller 102 receives an indication the cooling demand at 206has been satisfied. This indication may be the removal of a signalrepresenting the cooling demand received at 206. A thermostat orbuilding energy management system may remove the signal. For example, ahome thermostat stops the cooling demand signal when the temperaturemeasured by the thermostat reaches a programmed set-point. Typically,the cooling demand at 202 is satisfied, and a signal representing itremoved, at the same time the cooling demand at 206 is satisfied.

At 214, in response to the indication at 212, controller 102 returns thefree cooling target temperature to its original value. If the coolingdemands at 202 and 206 have both been met, controller 102 may also endfree and mechanical cooling. Method 200 may begin again at 202 whencontroller 102 receives another cooling demand.

In method 200, the reduced free cooling target temperature prevents thedischarge air from becoming undesirably cold. The economizing functionalso continues to utilize outdoor air, conserving energy. Outdoor airdamper 106 can remain fully open until the discharge air temperaturefalls below the reduced free cooling target temperature, rather than thehigher original free cooling target temperature.

It is noted that the embodiments disclosed are illustrative rather thanlimiting in nature and that a wide range of variations, modifications,changes, and substitutions are contemplated in the foregoing disclosureand, in some instances, some features of the present invention may beemployed without a corresponding use of the other features. Many suchvariations and modifications may be considered desirable by thoseskilled in the art based upon a review of the foregoing description ofvarious embodiments.

1. A method of cooling a supply of air, the method comprising:receiving, by a controller, a cooling demand; adjusting, by thecontroller, an economizer to achieve a first free cooling targettemperature as a discharge air temperature; receiving, by thecontroller, an additional cooling demand; reducing the first freecooling target temperature to achieve a second free cooling targettemperature; adjusting, by the controller, the economizer to achieve thesecond free cooling target temperature as the discharge air temperature;and performing mechanical cooling on a supply of air received from theeconomizer and adjusting the economizer to achieve the second freecooling target temperature by regulating an amount of outdoor air drawninto a heating, ventilation, and air conditioning (HVAC) unit.
 2. Themethod of claim 1, wherein the first free cooling target temperature is55° Fahrenheit and the second free cooling target temperature is 45°Fahrenheit.
 3. The method of claim 1, wherein the second free coolingtarget temperature is less than or equal to 45° Fahrenheit.
 4. Themethod of claim 1, further comprising: receiving, by the controller, anindication that the additional cooling demand has been met; andresponsive to the indication that the additional cooling demand has beenmet, returning, by the controller, the second free cooling targettemperature to the first free cooling target temperature.
 5. The methodof claim 1, wherein the mechanical cooling comprises moving the supplyof air past an evaporator coil.
 6. The method of claim 5, whereinrefrigerant within the evaporator coil absorbs heat from the supply ofair.
 7. The method of claim 1, wherein the economizer comprises anoutdoor air damper and a return air damper.
 8. The method of claim 7,wherein a combination of air from the outdoor air damper and the returnair damper forms the supply of air during the mechanical cooling.
 9. Acontroller of a heating, ventilation, and air conditioning (HVAC) unitfor cooling a supply of air, the controller being configured to: receivea cooling demand; adjust an economizer to achieve a first free coolingtarget temperature as a discharge air temperature; receive an additionalcooling demand; reduce the first free cooling target temperature toachieve a second free cooling target temperature; adjust the economizerto achieve the second free cooling target temperature as the dischargeair temperature; and perform mechanical cooling on a supply of airreceived from the economizer and adjust the economizer to achieve thesecond free cooling target temperature by regulating an amount ofoutdoor air drawn into the HVAC unit.
 10. The controller of claim 9,wherein the first free cooling target temperature is 55° Fahrenheit andthe second free cooling target temperature is 45° Fahrenheit.
 11. Thecontroller of claim 9, wherein the second free cooling targettemperature is less than or equal to 45° Fahrenheit.
 12. The controllerof claim 9, wherein the controller is further configured to: receive anindication that the additional cooling demand has been met; andresponsive to the indication that the additional cooling demand has beenmet, return the second free cooling target temperature to the first freecooling target temperature.
 13. The controller of claim 9, wherein themechanical cooling comprises moving the supply of air past an evaporatorcoil.
 14. The controller of claim 13, wherein refrigerant within theevaporator coil absorbs heat from the supply of air.
 15. The controllerof claim 9, wherein the economizer comprises an outdoor air damper and areturn air damper.
 16. The controller of claim 15, wherein a combinationof air from the outdoor air damper and the return air damper forms thesupply of air during the mechanical cooling.